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Extreme heat has engulfed southern Ontario and much of the U.S. Meteorologists are blaming an oppressive "heat dome."

Environment and Climate Change Canada has issued heat alerts for all of southern Ontario. In much of the region from Windsor to Toronto, it has forecast a high of 33 C with humidex values making it feel like 40 to 45. As of 11.25 a.m. ET, the temperature had already hit 30.8 C at Toronto Pearson International Airport.

Things are even worse in the U.S., especially the central and southwest regions, where highs are forecast to range from 43 to 46 C today even without taking the humidity into account, according to the National Weather Service, which has issued heat alerts for more than a dozen states.

"The air is forced to sink back to the surface, warming even further on the way," explains the U.S. National Oceanic and Atmospheric Administration's National Weather Service Prediction Centre.

To make matters worse, when air sinks, it tends to compress, causing it to heat up even more, says CBC meteorologist Johanna Wagstaffe, "and you have a recipe for a serious heat wave."

She added that in Canada, we're getting a lot of heat coming up from the U.S. in the south because of the clockwise motion of the hot air around a high pressure region.

'Not a meteorological term'

Marie-Eve Giguère, warning preparedness meteorologist with Environment Canada, said not all heat waves are associated with heat domes — some last just a couple of days before being pushed aside by a cold front.

She said that there's no actual threshold for how long a hot air mass has to hang around before it's considered a heat dome: "It's not a meteorological term at all."

She added: "This heat dome of hot and humid air that comes from a subtropical high that's stationary over the central U.S., it moves over us, and it's just not going anywhere as far as meteorologists can see."

She likened it to the summer equivalent to the polar vortex that traps cold air in southern Canada in the winter when we're north of the fast-moving, storm-bearing air current in the upper atmosphere called the jet stream.